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How to disable line numbers in the left margin?

Line numbers are disabled by default (ie. if there is no profile or when starting with --no-profile). If you have a ~/.teco_ini based on the included fallback.teco_ini, comment out or delete the following block (line 25):

    ! Set up margins !
    [_:M[lexer.test.woman]]_"F
      33ESTEXTWIDTH$9$U.w
      5*Q.w,0ESSETMARGINWIDTHN$$$
      Q.w,2ESSETMARGINWIDTHN$$$
    '

Editing files with very long lines is slow!?

By default, Scintilla caches the layout only of the current line. This is very inefficient if a line is on the screen that has a very costly layout calculation, e.g. an excessively long line. You can work around this issue by caching the entire document's layout and enabling threading by adding the following to the ED-hook under the !add! label in ~/.teco_ini (for instance after line 16 in fallback.teco_ini):

3@ES/SETLAYOUTCACHE//$
1000@ES/SETLAYOUTTHREADS//$

Also, make sure that your binary is built with Link-Time Optimizations (-O3 -flto). This is already the case for most regular releases and nightly builds.